k256/p256: add Arithmetic trait impls

[tarcieri]

See: RustCrypto/traits#219

[codecov-commenter]

Codecov Report

Merging #89 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   53.19%   53.13%   -0.06%     
==========================================
  Files          18       18              
  Lines        3412     3412              
==========================================
- Hits         1815     1813       -2     
- Misses       1597     1599       +2     

Impacted Files	Coverage Δ
k256/src/arithmetic.rs	85.23% <ø> (ø)
p256/src/arithmetic.rs	82.71% <ø> (ø)
k256/src/arithmetic/scalar/scalar_4x64.rs	75.82% <0.00%> (-0.74%)	⬇️

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